1. Field of the Invention
This invention relates to Ion Mobility Spectrometers and more particularly to extending the dynamic range of the ion mobility spectrometer by using an input sample air mixer which is controlled by an electrical signal.
2. Description of the Prior Art
Present ion mobility spectrometers have limited dynamic range and are easily saturated when the sample molecules exceed the number of reaction ions available to ionize the sample molecules. Normal sensitivities of ion mobility spectrometers is in the range of parts per billion. An ionizer in the reaction region generates reactant ions over a period of time which in turn are attracted by the affinity of the sample molecule which removes charge from the reactant ions to itself. The ionized sample molecules are then detected by their drift time in a drift region. A typical range of concentrations of the sample is less than 100 to 1.
U.S. Pat. No. 4,259,573 which issued on Mar. 31, 1981 to J. M. Prober et al., describes an accurate method of determining small concentrations of chemical compounds by plasma chromotography "Ion Mobility Spectrometry". Small, known increments of species A plus, optionally, another calibrant species are introduced in turn into the plasma chromotograph together with the unknown sample chemical and the respective changes of the amplitude or area of a characteristic ion peak of species A in the unknown sample are measured.
U.S. Pat. No. 4,311,669 which issued on Jan. 19, 1982 to G. E. Spangler describes a sample inlet port of an ion mobility detector having a membrane interface. The sample which is included as a vapor component in a gas stream impinges on the exterior surface of the membrane and penetrates the membrane and is carried into the ion mobility spectrometer by means of a carrier gas which scrubs the interior surface of the membrane.
U.S. Pat. No. 4,551,624 which issued on Nov. 5, 1985 to G. E. Spangler et al. describes an ion mobility spectrometer system wherein a reagent such as acetone and/or carbon tetrachloride is injected into the carrier gas prior to entering the reaction region. The reagent has a higher proton affinity, electron affinity, or acidity than contaminants in the sample gas, a lower proton affinity, electron affinity, or acidity than at least 1 constituent of the sample gas to be detected, and does not cluster with water disposed in the reagent source.